I recently heard an urban legend-like story about a guy in a developing country who decided engine oil was too expensive and so used some vegetable oil instead. The story says the car ran for some time and then it's engine got broken and required complete overhaul so that guy wasted much more than what he saved on oil.

I'm not a car owner, so I'm really curious - what's the worst thing that can typically happen if some unsuitable oil is used instead of engine oil?

Maybe this is one for Mythbusters? dsc.discovery.com/tv/mythbusters (Don't think they've done this specifically; Obviously, I think the vegetable oil work for a day if you're lucky, but it'd be cool to see them break it down on what specifically happens...
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NSGodApr 14 '11 at 1:10

The worst thing that can happen is that you can destroy your vehicle. Your vehicle is designed to use some very specific lubrication, and using something other than what's specified can be detrimental.

If the lubricant cannot withstand high temperatures, you could gunk up your engine and require a rebuild.

The oil in the crankshaft is in contact with air and gasoline fumes. If you used an oil with an ignition point as low as the engine temperature, it would probably start a fire in there.

I'm not sure how vigorous the fire would be (that likely depends on the air supply, which probably varies between engines), but it would eventually deplete the oil, cover the crankshaft in residue, and destroy the headgasket.

That said, there are a lot of kinds of vegetable oil, including standard motor oil which comes from castor beans.